The invention relates to a high-pressure hydraulic system having a valve for varying the flow of hydraulic oil between a pump and a hydraulic load.
Such high-pressure hydraulic valves, in various types and constructions, are employed in mobile hydraulic systems. Examples of such different types are proportional valves, load holding brake valves, and blocking/safety valves. The mobile hydraulic systems are components of dredgers, cranes and other lifting devices, for instance. In such applications, the weight of the hydraulic units plays a role. The weight should be as low as possible.
Typically, the housings of high-pressure hydraulic valves are made from gray cast iron or spherulitic graphite iron, and the internal parts, some of them movable, are made from hardened and tempered steel. The weight of the high-pressure hydraulic valve is determined substantially by the weight of this housing. One such high-pressure hydraulic valve is known from WO92/07208. In U.S. Pat. No. 5,556,075 that matured from this application, it is mentioned as an essential effect of the invention that the weight is quite low. Thus in both of these previously known applications it is proposed that a part of the valve spindle, or the entire valve spindle and valve cone assembly, be produced from a ductile material, such as copper, brass, or in particular aluminum. This is intended to improve the resistance to leakage. In the case where aluminum is used, a reduction in weight is indeed achieved, but in view of a housing whose mass predominates, this reduction is only minimal. A weight reduction for the valve spindle and valve cone assembly also leads to a lower inertial mass and thus to a lesser dynamic pressure loss in periodic switching. Because of the circumstance already mentioned, that the weight of the valve spindle and valve cone assembly is only a small proportion of the total weight, the saving in weight for the high-pressure hydraulic valve intrinsically is not very significant. Furthermore, there is the problem of the different coefficients of thermal expansion of the various materials.
The goal of making machine components as light in weight as possible has existed for a long time. Hence lightweight structural materials have long been employed in a great many fields of industry. As an example, it can be mentioned that such lightweight materials as titanium and aluminum are used in passenger-carrying motor vehicles as well. This is known for instance from U.S. Pat. No. 5,169,460. Since as noted, there has long been a need to reduce weight, it can be concluded that in industrial fields in which such lightweight materials have not yet been used, strong arguments exist that preclude the use of such materials.
The object of the invention is to reduce the weight of a high-pressure hydraulic valve markedly further, without adversely changing its utility. Advantageous refinements will become apparent from the dependent claims.
The total weight of a high-pressure hydraulic valve is definitively determined by its housing. Such shape-related data as size and wall thickness, on the one hand, and the specific weight of the material used on the other, play a significant role. The size of the housing is influenced for instance by the size of the flow through it that is to be controlled, as well as by the structural design that is associated with the mode of operation. The maximum pressure that the high-pressure hydraulic valve has to control also has a determining influence on the housing size, for instance on the minimum thickness of walls. Hydraulic systems that can be classified as high-pressure systems are characterized by operating pressures of more than 100 bar. Systems with a rated operating pressure of 420 bar are known. In this respect it should be noted that precisely in such mobile systems as cranes and dredges, peak loads can certainly occur that exceed the rated operating pressure. Although as a rule safety devices that are intended to prevent greater loads are present, nevertheless even in such cases, dynamic peak loads can occur. The manufacturer of high-pressure hydraulic valves must therefore provide proof that the function and durability of the unit is assured even at pressures that are a multiple of the rated operating pressure, for instance four times the rated operating pressure.
In contrast to the time-tested prior art, in which the gray cast iron or preferably spherulitic graphite iron GGG 40 is used as the housing material and hardened steel is used for the internal parts, it is proposed according to the invention that a precipitation-hardenable wrought aluminum alloy, such as AlZnMgCu, be used as the housing material. Since the specific weight of such an alloy is approximately 2.8 g/cm3, while that of spherulitic graphite iron GGG 40 has a value of 7.25 g/cm3, a saving in weight for the weight of the housing, assuming the same dimensions, of about 60% is attainable.
Often, housing parts for high-pressure hydraulic valves are made not from molded castings but rather from solid blocks. On the one hand this is because the numbers of pieces are so low that producing molds and mold cores for the internal hollow spaces is unfeasible because of the high cost, and on the other because often a plurality of parts are combined as elements that can be lined up to make larger units. Here there are advantages if the housings have a blocklike shape. In such blocklike housings, the absolute weight advantage, expressed in kilograms per component, is especially significant when a precipitation-hardenable aluminum alloy is used instead of spherulitic graphite iron GGG 40.
The mechanical specifications for the above materials, such as tensile strength, Brinell hardness, modulus of elasticity, and tensile yield strength, have long been known. Although the tensile strength and the tensile yield strength of precipitation-hardenable aluminum alloys are for instance higher than the corresponding values for spherulitic graphite iron GGG 40, the profession has long shied away from using precipitation-hardenable aluminum alloys as material for the housings of high-pressure hydraulic valves.
What evidently also stood in the way of the solution according to the invention was that the wear resistance of spherulitic graphite iron GGG 40 is considered to be extraordinarily high, while aluminum alloys in general are not considered especially abrasionproof. However, as tests have shown, in hydraulic equipment this aspect lacks overriding significance, because the hydraulic oil present in such equipment acts as a lubricant in the event of sliding stress. As an advantageous feature of the invention, however, a surface treatment of the components made from the aluminum alloy can be done, preferably an anodic oxidation. Such methods have already long been known per se. One advantageous method for the intended purpose is the creation, by the method known as HART-COAT(copyright) (trademark of the corporation doing business as AHC-Oberflxc3xa4chentechnik, Friebe and Reininghaus GmbH), of a coating with a comparatively very slight pore volume. In this method, the anodic oxidation takes place in a specially composed acid electrolyte, which is cooled during the process.